British reviewers who have tested the Apple (AAPL) iPhone that goes on sale in the U.K. Friday report that it comes pre-installed with a software update — 1.1.2 — that disables third-party applications.

According to the British gadget website T3, the update closes the so-called TIFF exploit — the software loophole used by hackers to “jailbreak” version 1.1.1. This loophole allowed iPhone owners to install dozens of third-party apps, including such popular add-ons as Navizon (a location finder), Voice Notes (a voice recorder) and instant-messaging programs like Apollo and Mobile Chat.

Thanks to one-click installations scripts like AppSnapp, these unauthorized iPhone add-ons have become almost mainstream. AppSnapp’s developers report that their software was downloaded 144,000 times in its first three days — which suggests that as many as 1 in 10 iPhone owners could be in for a rude surprise when they upgrade their software next weekend.

Some Apple bloggers — led by Quincy Pince-Nez at 9to5 Mac — advocate holding-off any iTunes and iPhone updates until programmers can find another way to install their apps. Apple would undoubtedly prefer that everybody wait until it releases its official iPhone software developers kit (SDK) in February, and Apple-sanctioned apps start to flow in.

The update is also likely to disable — and perhaps re-brick — iPhones unlocked to work with cellular providers other than Apple’s official carriers (AT&T in the U.S., O2 in the U.K., T-Mobile in Germany and Orange in France).

[Photo courtesy of T3]